CMS-SMP-19-002

CMS-SMP-19-002 ; CERN-EP-2021-008
Measurement of W$\gamma$ production cross section in proton-proton collisions at $\sqrt{s} = $ 13 TeV and constraints on effective field theory coefficients
CMS Collaboration
3 February 2021
Phys. Rev. Lett. 126 (2021) 252002
Abstract: A fiducial cross section for W$\gamma$ production in proton-proton collisions is measured at a center-of-mass energy of 13 TeV in 137 fb$^{-1}$ of data collected using the CMS detector at the LHC. The $\mathrm{W} \to \mathrm{e}\nu$ and $\mu\nu$ decay modes are used in a maximum-likelihood fit to the lepton-photon invariant mass distribution to extract the combined cross section. The measured cross section is compared with theoretical expectations at next-to-leading order in quantum chromodynamics. In addition, 95% confidence level intervals are reported for anomalous triple-gauge couplings within the framework of effective field theory.
Links: e-print arXiv:2102.02283 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; CADI line (restricted) ;

Figures & Tables	Summary	References	CMS Publications

Figures
png pdf	Figure 1: Representative Feynman diagram for ${\mathrm{p}} {\mathrm{p}} \to \ell ^+\nu _{\ell}\gamma $ production with a TGC vertex.
png pdf	Figure 2: Expected and observed distributions in the invariant mass of the lepton-photon system in the electron (left) and muon (right) channels. The signal and background processes that are drawn correspond to the estimates made before the fit, except that normalization of the electron-induced photon (one of the free parameters) is scaled by 1.8 from its pre-fit value. The uncertainty in the prediction (the hatched band) is the quadratic sum of the systematic uncertainties. The uncertainty in the data is statistical. The W$\gamma$ refers to the MadGraph 5_aMC@NLO simulation of W$\gamma$ events.
png pdf	Figure 2-a: Expected and observed distributions in the invariant mass of the lepton-photon system in the electron channel. The signal and background processes that are drawn correspond to the estimates made before the fit, except that normalization of the electron-induced photon (one of the free parameters) is scaled by 1.8 from its pre-fit value. The uncertainty in the prediction (the hatched band) is the quadratic sum of the systematic uncertainties. The uncertainty in the data is statistical. The W$\gamma$ refers to the MadGraph 5_aMC@NLO simulation of W$\gamma$ events.
png pdf	Figure 2-b: Expected and observed distributions in the invariant mass of the lepton-photon system in the muon channel. The signal and background processes that are drawn correspond to the estimates made before the fit, except that normalization of the electron-induced photon (one of the free parameters) is scaled by 1.8 from its pre-fit value. The uncertainty in the prediction (the hatched band) is the quadratic sum of the systematic uncertainties. The uncertainty in the data is statistical. The W$\gamma$ refers to the MadGraph 5_aMC@NLO simulation of W$\gamma$ events.
png pdf	Figure 3: The photon $ {p_{\mathrm {T}}} $ distribution used for the extraction of limits on dimension-six EFT operators. The expected yields correspond to the estimates made before the fit. The uncertainty in the prediction (the hatched band) is the quadratic sum of the systematic uncertainties. The uncertainty in the data is statistical. The last bin includes the overflow.

Tables
png pdf	Table 1: Expected and observed numbers of events. The signal and background yields correspond to the estimates made before the fit, except that normalization of the electron-induced photon yield (one of the free parameters) is scaled by 1.8 from its pre-fit value. The uncertainty is the quadratic sum of the systematic uncertainties. The W$\gamma$ refers to the MadGraph 5_aMC@NLO simulation of W$\gamma$. The W$\gamma$ signal and W$\gamma$ nonfiducial are the contributions to the signal region from the W$\gamma$ process originating from within and outside the fiducial region, respectively.
png pdf	Table 2: Expected and observed 95% confidence level limits on four dimension-six operator coefficients. The units of the limits are TeV$^{-2}$.

Summary

In summary, the cross section for $\mathrm{pp} \to \mathrm{W}\gamma$ production has been measured at a center-of-mass energy of 13 TeV for the first time. The measured cross section in a defined fiducial region is $\sigma = $ 15.58 $\pm$ 0.05 (stat) $\pm$ 0.73 (syst) $\pm$ 0.15 (theo) pb $=$ 15.58 $\pm$ 0.75 pb, consistent with the MadGraph 5_aMC@NLO next-to-leading-order (NLO) quantum chromodynamics (QCD) prediction of $\sigma = $ 15.4 $\pm$ 1.2 (scale) $\pm$ 0.1 (PDF) pb, and less than the POWHEG NLO QCD with NLO competition prediction of $\sigma = $ 22.4 $\pm$ 3.2 (scale) $\pm$ 0.1 (PDF) pb. The cross sections in the electron and muon channels are consistent with each other. The high tail of the photon transverse momentum distribution is used to set 95% confidence level limits on dimension-six effective field theory parameters, including the most stringent limit to date on the coefficient of $\mathcal{O}_{WWW}$, the lowest dimension $CP$-even operator that directly alters the $\mathrm{W}\mathrm{W}\gamma$ TGC.

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